Hematopoiesis is the process by which blood cells develop and differentiate from pluripotent stem cells in the bone marrow. This process involves a complex interplay of polypeptide growth factors (cytokines) acting via membrane-bound receptors on the target cells. Cytokine action results in cellular proliferation and differentiation, with response to a particular cytokine often being lineage-specific and/or stage-specific. Development of a single cell type, such as a platelet or a neutrophil, from a stem cell may require the coordinated action of a plurality of cytokines acting in the proper sequence.
The known cytokines include the interleukins, such as IL-1, IL-2, IL-3, IL-6, IL-8, etc.; and the colony stimulating factors, such as G-CSF, M-CSF, GM-CSF, erythropoietin (EPO), etc. In general, the interleukins act as mediators of immune and inflammatory responses. The colony stimulating factors stimulate the proliferation of marrow-derived cells, activate mature leukocytes, and otherwise form an integral part of the host's response to inflammatory, infectious, and immunologic challenges.
Various cytokines have been developed as therapeutic agents. For example, erythropoietin, which stimulates the development of erythrocytes, is used in the treatment of anemia arising from renal failure. Several of the colony stimulating factors have been used in conjunction with cancer chemotherapy to speed the recovery of patients' immune systems. Interleukin-2, .alpha.-interferon and .gamma.-interferon are used in the treatment of certain cancers. Factors responsible for stimulation of megakaryocytopoiesis and thrombocytopoiesis resisted definitive characterization, due in part to lack of a reliable source, a lack of convenient assays, and a lack of knowledge as to the site(s) of production until recently, despite three decades of work to isolate and characterize them. The megakaryocytopoietic factor referred to in the literature as "thrombopoietin" (recently reviewed by McDonald, Exp. Hematol. 16:201-205, 1988; and McDonald, Am. J. Ped. Hematol. Oncol. 14:8-21, 1992) has now been identified and isolated (see copending U.S. patent application Ser. No. 08/252,491; Lok et al., Nature 369:565-568, 1994; and Kaushansky et al., Nature 369:568-571, 1994; all herein incorporated by reference).
Mild bleeding disorders (MBDs) associated with platelet dysfunctions are relatively common (Bachmann, Seminars in Hematology 17: 292-305, 1980), as are a number of congenital disorders of platelet function, including Bernard-Soulier syndrome (deficiency in platelet GPIb), Glanzmann's thrombasthenia (deficiency of GPIIb and GPIIIa), congenital afibrinogenemia (diminished or absent levels of fibrinogen in plasma and platelets), and gray platelet syndrome (absence of .alpha.-granules). In addition there are a number of disorders associated with platelet secretion, storage pool deficiency, abnormalities in platelet arachidonic acid pathway, deficiencies of platelet cyclooxygenase and thromboxane synthetase and defects in platelet activation (reviewed by Rao and Holmsen, Seminars in Hematology 23: 102-118, 1986). At present, the molecular basis for most of these defects is not well understood.
Leukopenia is a deficiency in one or more of the cell-types collectively known as white blood cells (WBCs). Included are cells from the myeloid and lymphoid lineage. Lymphoid cells are T-cells and B-cells and their progenitors. Myeloid lineage cells include: the granulocyte/macrophage (GM) progenitor cells, which through the process known as differentiation, mature into neutrophils, monocytes, macrophages, eosinophils and basophils. The myeloid lineage white blood cells generally function as phagocytes and remove foreign material (e.g. fungi, bacteria and virus) by digestion using enzymes that are either released extracellularly or by endocytosis and degradation inside the cell.
A WBC count generally measures all the WBCs and a normal value is in the range of 5.0 to 10.0.times.10.sup.9 /liter. Clinically significant deficiencies of WBCs are usually deficiencies in neutrophils (neutropenia) or lymphocytes (lymphocytopenia). Levels of neutrophils and lymphocytes can be identified by differential white blood cell counts which evaluate each cell type in the WBC fraction separately. Neutropenia is diagnosed in patients with neutrophil levels below 2.0.times.10.sup.9 /liter. For discussion of the clinical aspects of neutropenia, see, for example, Cecil Textbook of Medicine (Wyngaarden et al. (eds.), 19th edition, W.B. Saunders Co., Philadephia, Pa., 1992). Neutrophil levels below 0.5.times.10.sup.9 /liter are considered life-threatening, resulting in serious recurrent and often difficult-to-treat infections. Abnormalities in the bone marrow account for the majority of clinically significant neutropenias and are primarily caused by cytotoxic drugs, radiation, marrow invasion by abnormal cells and infections.
While many cytokines have been characterized and some have proven clinical applications, there remains a need in the art for additional agents that stimulate proliferation and differentiation of myeloid and lymphoid precursors and the production of mature blood cells. There is a particular need for agents that stimulate the development and proliferation of cells of the multiple myeloid lineage. There is a further need in the art for agents that can be used in the treatment of cytopenias, including thrombocytopenia (low number of circulating platelets, less than about 1.times.10.sup.5 platelets/mm.sup.3), anemias and leukopenias, particularly neutropenia, and other platelet disorders. The present invention fulfills these needs and provides other, related advantages.